I've noticed this trend, as well. Liberal-leaning folks tend to be a lot more supportive of Islam (in all its forms) than of Christianity; for example, they tend to want me to make the "extremist vs. moderate" distinction for Islam, but not for Christianity. And I've had people get so angry with me --- for merely saying that Islam should be subject to the same level (not more, not less, but the same level) of criticism that we direct towards Christianity --- that they refuse to speak to me about it any more.
I find this tendency more than a little frustrating; I can hold an intelligent discussion with this sort of person about what, exactly, we find wrong with Christianity, but the minute the conversation turns towards Islam, I'm suddenly this "conservative bigot" for comparing Islam to Christianity with regard to some of its specific tenets (amazing how one topic can change me from being a "tree-hugging lib" to a "bigot conservative"). If you disagree with my comparisons, that's fine, but I'm surprised by how many (non-Muslim) people seem suddenly unwilling to continue a reasonable discussion about something once I criticize Islam. And it's not even Muslim people that I'm criticizing, it's Islam itself --- the religion.
Another problem I've noticed is how any criticism of Islam is automatically associated with "racism" against Arabs. I've criticized some of Islam's specific religious tenets before (such as how they treat women, complete with Quran quotations to support my criticisms), never once referring to Arabs or Arabic/middle-eastern culture apart from Islam, and yet I've been immediately dismissed from the discussion as a "racist" or a "bigot." And once I made the comment that I don't believe Islam should be magically exempt from any criticism on the grounds that "criticizing Islam is racist," and I was similarly dismissed. It's really interesting, how the American left is so willing to go above and beyond the normal to defend Islam where they won't defend Christianity.
Don't get me wrong, I find them both abhorrent and I criticize them both regularly (and I'm probably guilty of ranking on Christianity moreso than on Islam, even if for no other reason than because I deal with it more regularly than Islam). But this attitude is even prevalent among the atheist community, which (in most other cases) prides themselves about being "more reasonable" or "more rational" than everyone else. And yet, they suddenly become incapable of even *trying* to provide a reasonable basis to defend Islam when it is criticized, instead opting to call its critics "racist" off the bat. It's a discussion-stopper, and an annoying one at that.
[inb4 "lol u racist!]
For all the things that I never did
For all the places I never was
For all the people I never stopped
But there was nothing I could do..."